1. Technical Field
The present disclosure generally relates to electronic circuits, and in particular to the forming of a circuit enabling to controllably retain electric charges for a time measurement. It more specifically relates to the forming of an electric charge flow element.
2. Discussion of the Related Art
In many applications, it is desired to have information representative of a time elapsed between two events, be it an accurate or approximate measurement. An example of application relates to the time management of rights of access, especially to media.
The obtaining of such information representative of the elapsed time conventionally requires a time measurement by an electronic circuit powered, for example, by means of a battery, to avoid losing the information variation when the circuit is not being used.
It would be desirable to have a time measurement which works even when the electronic measurement circuit is not being powered.
An electronic device where the time elapsed between two events is determined by measuring the charge of a capacitive storage element having an electrode connected to an electrode of a capacitive charge flow element having a leakage in its dielectric space has already been provided, for example, in U.S. Pat. No. 8,331,203. The storage element is charged when the device is powered, and its residual charge, after interruption of the power supply, is measured when the device is powered again. This residual charge is considered as representative of the time period elapsed between the two times of power supply of the device.
The charge flow element comprises a region of smaller thickness in its dielectric space, capable of letting charges leak by tunnel effect. The storage element discharge speed depends on the dimensions of the leakage region of the flow element. In particular, the storage element discharge speed increases when the thickness of the leakage element decreases and/or when the surface (in top view) of the leakage region increases.
A disadvantage is that, in practice, the sizing of the leakage region is strongly dependent on the considered technological manufacturing process. Indeed, the charge retention circuit is generally integrated to a chip comprising other components, for example, memories, logic blocks, etc. To avoid increasing the chip manufacturing cost, it is desired to form the charge retention circuit with no additional manufacturing step with respect to the steps of manufacturing of the other components. In certain recent technological processes, the dielectric layers available to form the leakage region of the flow element are too thin to enable a slow discharge of the storage element, even when the surface area of the leakage region is decreased to a minimum. As a result, the time measurement may only work, in the absence of power supply, during a very short time, poorly adapted to most applications.